Staff: Mentor

It means that the particle is equally likely to be found on one side of the origin of your coordinate system, as on the opposite side. Whether that's possible or not depends on the situation. For a hydrogen atom in the ground state, with the proton at the origin, <x> for the electron is in fact zero. For the classic textbook particle in an "infinite square well" whose boundaries are at x = 0 and x = L, <x>= 0 is not possible.

Thanks, that really helps. I was wondering because I was looking at a question that didn't really specify the conditions. I'll check through my working and see if I've made any mistakes. Also, if <x> is zero, can <p> also be zero?